This invention pertains to implantable medical devices such as cardiac pacemakers and implantable cardioverter/defibrillators. In particular, the invention relates to a system and method enabling an implantable medical device to communicate information to an external data logging device.
Implantable medical devices are commonplace today, particularly for treating cardiac dysfunction. Cardiac pacemakers, for example, are implantable medical devices that replace or supplement a heart""s compromised ability to pace itself (i.e., bradycardia) due to chronotropic incompetence or a conduction system defect by delivering electrical pacing pulses to the heart. Implantable cardioverter/defibrillators (ICD""s) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates (tachycardia) including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device.
Most pacemakers today are operated in some sort of synchronous mode where the pacing pulses are delivered in a manner that is dependent upon the intrinsic depolarizations of the heart as sensed by the pacemaker. ICD""s must also sense the electrical activity of the heart in order to detect an arrhythmia that will trigger delivery of the shock pulse in an attempt to reverse the condition. Such sensed information can be stored by the device in the form of a data log which can be transferred later to an external programmer via a radio link. Due to the limited data storage capacity in a typical implanted device, however, only a small fraction of the total sensed information is actually stored in the data log. One way to circumvent this problem would be to transmit the data log continuously or at frequent intervals to the external programmer. An implantable device has only a battery power supply, however, and the energy costs of such frequent radio transmissions would be excessive. What is needed is a low-energy data transmission method so that the implantable device can transmit logged data either continuously or at frequent intervals to an external device. It is toward this objective that the present invention is primarily directed.
The present invention relates to a data logging system and method in which an implantable medical device transmits data to an external data logging device with low energy potential signals. The implantable device generates potential signals by operating a current source to cause corresponding electrical potentials that can be sensed at the skin surface by the external data logging device. The current source is operated so as to generate a carrier waveform that can be modulated with digitally encoded information corresponding to sensed data logged by the implantable device. The external data logging device includes electrodes at the skin surface for sensing potentials and circuitry for demodulating the sensed carrier waveform to derive the encoded logged data. The data logging device may then store the digital data in a storage medium for later retrieval. The storage medium is preferably a non-volatile memory that can be removed and replaced at periodic intervals. Alternatively, the contents of the storage medium can be directly transferred to another device at periodic intervals.
In accordance with the invention, the carrier waveform is digitally modulated with the digitally encoded information by varying the amplitude or frequency of the carrier waveform using, for example, amplitude shift-keying or frequency shift-keying. In a particular embodiment, a digital pulse train is modulated with the digitally encoded information by varying the frequency, width, or position of the pulses. The pulse train is then used to amplitude modulate the carrier waveform.
Certain implantable medical devices, such as rate-adaptive pacemakers, may use an impedance technique for measuring minute ventilation and/or cardiac stroke volume. In that technique, an oscillating current is made to flow between two electrodes located within the thorax, and the impedance between the electrodes is measured. In accordance with the invention, the impedance measuring current may be used as the carrier waveform and modulated with digitally encoded information by the implantable device for transmission to the external data logging device.
The present invention may be incorporated into a system where the implantable device is a cardiac device such as an implantable cardioverter/defibrillator, cardiac pacemaker, or combination device. In one embodiment, the data logging device is a self-contained unit in the form of a patch designed for external affixation to a patient""s skin surface at a suitable location.